Focused ion beam technology (FIB) utilizes an apparatus that focuses an ion beam from an ion source through a lens and irradiates the beam onto a sample. In the fabrication of integrated circuits, FIB is frequently used to mill away (etch) material by irradiating an ion beam of relatively high current onto the substrate. The focused ion beam can be directed to a very small point on a semiconductor device and then scanned, raster fashion, over a surface where material is to be removed. As an ion impinges on the semiconductor device surface, its momentum is transferred, resulting in the removal of one or more surface atoms according to a process called sputtering. By selecting a raster pattern of a given overall shape, for example a horizontal raster pattern, a correspondingly shaped area of surface material can be removed. Often several successive layers of a semiconductor device are removed in a given area in order to reach and possibly sever an underlying layer.
The rate and controllability of milling can be enhanced by injecting gases that preferentially mill particular materials, such as dielectric or conductive materials. Gases are injected near the surface of the semiconductor device during the milling process to increase the efficiency of removing a specific type of material. As the boundaries between different materials are traversed, the type of gas injected may be changed to conform to the requirements of the new material; that is, a different gas may be used for each material or class of materials. Such techniques can be used to selectively expose the integrated circuit structure for probing or examination, cut holes through power and ground planes, and to selectively sever conductors. For example, U.S. Pat. Nos. 5,188,705 and 5,376,791 to Swanson et al disclose the use of a focused ion beam for sputtering (etching) of semiconductor devices while directing iodine vapor toward the surface to enhance the removal of materials such as silicon and aluminum. See also U.S. Pat. No. 5,009,743 to Swann, which describes the use of dual ion guns in combination with injection of molecular iodine, and U.S. Pat. No. 4,226,666 to Winters et al. which describes etching employing electron-beam or ion-beam radiation and a noble gas halide such as XeF2, XeF4, XeF6, KrF2, KrF4 and KrF6. The use of XeF2 with FIB for preferential etching of dielectric in semiconductor devices has become commonplace as the use of XeF2 substantially increases the etching rate of dielectric relative to the etching rate of most metals so that conductors can be exposed rapidly and with less risk of electrostatic discharge damage.
For a number of well known reasons, integrated circuits are now transitioning from aluminum to copper interconnects as device generation goes beyond the 0.35 micron design rules. For preferential FIB etching of aluminum metal relative to dielectric, the use of fluorine, chlorine or iodine gas is typically used to increase the etching rate. However, it has been found that the use of halogens, in general, causes severe corrosion of copper surfaces due to the high reactivity of halogens with copper. Moreover, the use of FIB for removal of copper is more complicated relative to aluminum due to the presence of copper grains. Current FIB processes for removal of copper result in the redeposition of copper.
A process for milling copper metal from a substrate having an exposed copper surface includes absorbing a halogen gas onto the exposed copper surface to generate reaction products of copper and the halogen gas; removing unreacted halogen gas from the surface; and directing a focused ion beam onto the surface to selectively remove a portion of the surface comprising the reaction products. Preferably, unreacted halogen gases are removed from the exposed copper metal surfaces by an electron beam scan. The beam current of the focused ion beam is preferably from about 500 to 3,000 picoAmps. In a preferred embodiment, iodine is absorbed directly onto the exposed copper surfaces.
A process for focused ion beam milling multiple layers of a substrate, wherein the substrate comprises an insulating layer in contact with an underlying copper surface includes exposing the substrate to a noble gas halide within an enclosed chamber; directing a focused ion beam onto a portion of the insulating layer and removing the portion to expose the underlying copper surface; absorbing a halogen gas onto the exposed copper surface to generate reaction products of copper and the halogen gas; removing unreacted halogen gas from the surface; and directing a focused ion beam onto the surface to selectively remove a portion of the surface comprising the reaction products.
Other advantages and a fuller understanding of the invention will be had from the accompanying drawings and detailed description that follows.